TESTS FOR BLOOD. 
§ 42-1 
71 
added and allowed to cool ; in this way hsemin crystals have been 
obtained from a crowbar fifty days after having been blood-stained. 1 
3. Guaiacum Test. —This test depends upon the fact that a solution 
of haemoglobin develops a beautiful blue colour if brought into contact 
with fresh tincture of guaiacum and peroxide of hydrogen. The simplest 
way to obtain this reaction is to moisten the suspected stain with distilled 
water ; after allowing sufficient time for the water to dissolve out some 
of the blood constituents, moisten a bit of filter-paper with the weak 
solution thus obtained ; drop on to the moist space a single drop of 
tincture of guaiacum which has been prepared by digesting the inner 
portions of guaiacum resin in alcohol, and which has been already tested 
on known blood, so as to ascertain that it is really good and efficient for 
the purpose ; and, lastly, a few drops of peroxide of hydrogen. Dragen- 
dorff uses his borax solution, and, after a little dilution with water, adds 
the tincture and then Heunefeld’s turpentine solution, which is composed 
of equal parts of absolute alcohol, chloroform, and French turpentine, 
to which one part of acetic acid has been added. The chloroform 
separates, and, if blood was present, is of a blue colour. 
§ 42. To prove by chemical and physical methods that a certain 
stain is that of blood is often only one step in the inquiry, the next 
question being whether the blood is that of man or of animals. The 
blood corpuscles of man are larger than those of any domestic animal 
inhabiting Europe. The diameter of the average red blood corpuscle is 
about the of a millimetre, or 7*9 /x. 2 The corpuscles of man and 
of mammals, generally speaking, are round, those of birds and reptiles 
oval, so that there can be no confusion between man and birds, fishes, 
or reptiles ; if the corpuscles are circular in shape the blood will be 
that of a mammal. By careful measurements, Dr Richardson, of 
Pennsylvania, affirms that it is quite possible to distinguish human 
blood from that of all common animals. He maintains, and it is true, 
that, by using very high magnifying powers and taking much trouble, 
an expert can satisfactorily identify human blood if he has some half- 
dozen drops of blood from different animals—such as the sheep, goat, 
horse, dog, cat, etc.—all fresh at hand for comparison, and if the human 
blood is normal. However, when we come to the blood of persons suffer¬ 
ing from disease, there are changes in the diameter and even the form of 
the corpuscles which much complicate the matter ; while, in blood-stains 
of any age, the blood corpuscles, even with the most artfully contrived 
solvent, are so distorted in shape that he would be a bold man who should 
venture on any definite conclusion as to whether the blood was certainly 
human, more especially if he had to give evidence in a criminal case. 
1 Brit. Med. Journ., Feb. 17, 1894. 
2 stou °f an inch ; the Greek letter fi is the micro-millimetre, or 1000th of a 
millimetre, -00003937 inch. 
